Evolution of Vertebrate Limb Development

Abstract

The origin and diversification of fins and limbs have long been a focus of interest to both palaeontologists and developmental
biologists. Studies conducted in recent decades have resulted in enormous progress in the understanding of the genetic and
developmental bases of the evolution of paired appendages in vertebrates. These discoveries in the areas of genetics and developmental
biology have shed light on the mechanisms underlying the evolution of this key morphological innovation in vertebrates. In
this article, recent advances in these fields and how they can provide a mechanistic explanation for the origin and evolution
of paired appendages have been discussed.

Key Concepts

Step‐wise changes seem to be involved in the acquisition of paired fins, such as regionalisation of the lateral plate mesoderm
(LPM) into the anterior and posterior LPM; sub‐division of the LPM into somatic and splanchnic layers; acquisition of expression
of genes that initiate limb formation, such as Tbx4/5; and dorso‐ventral compartmentalisation of ectoderm.

Morphological changes during the fin‐to‐limb transition include the acquisition of the autopod and the evolutionary modification
of skeletal patterns along the anterior–posterior axis.

The fin‐to‐limb transition seems to involve changes in transcriptional regulation of HoxA and HoxD clusters, changes in the expression of Gli3 and Shh, loss of the fin fold, and modification of the BMP‐SOX9‐WNT Turing network.

Changes in the activity of regulatory elements of genes known to play pivotal roles in limb development seem to be related
to the morphological diversification of limbs and the loss of paired fins and limbs.

Keywords: limb; fin; evolution; vertebrates; lateral plate mesoderm

Figure 1. Models for the evolution of paired appendages in vertebrates. (a) The ‘lateral fin fold theory’, in which two paired appendages
evolved from a continuous lateral fin, was proposed by Thacher ; Mivart and Balfour . (b) The ‘pelvic before pectoral fin’ model suggested that the co‐option of collinear expression of Hox (green bars) from the body axis in paired appendages originated in the pelvic appendage. In this model, pelvic Hox expression was subsequently co‐opted for the development of the pectoral appendage (Tabin and Laufer, ). (c) The ‘pectoral before pelvic fin’ model was advocated based on fossil records and on the general anterior–posterior
gradient of development (Coates, ; Thorogood and Ferretti, ). (d) The molecular mechanisms of fin development in paired appendages have been proposed to be adopted from the median fins
(Freitas et al.,). This view was based on the observation that Hoxd genes (green bars) were expressed in a nested manner in the developing shark median fin, as seen in the paired appendages.
(e) Two alternative models for the evolution of paired appendages proposed by Ruvinsky and Gibson‐Brown highlighted the coevolution of the T‐box gene with Pitx1. Initially, an ancestral jawless vertebrate acquired a novel expression domain of Tbx4/5 (purple) in the lateral plate mesoderm at the pectoral level, and this led to the formation of the first pair of fins (top).
Then two alternative scenarios were considered: the T‐box cluster underwent duplication, and Tbx4 (red) and Tbx5 (blue) were co‐expressed in a single pair of fins (left middle). Subsequently, Tbx4 (red) acquired the novel expression domain in the body wall at the pelvic level (bottom). Alternatively, Tbx4/5 (purple) acquired a novel expression domain in the posterior part of the lateral plate mesoderm (right middle), and then
Tbx4/5 underwent duplication and gave rise to pectoral (blue) and pelvic fins (red) (bottom). According to this model, posteriorly
expressed Pitx1 (yellow) modified the identity of the posterior/pelvic appendages together with Tbx4. (f) Schematic model for regionalisation and differentiation of the ventral mesoderm and the lateral plate mesoderm (LPM)
in amphioxus, lampreys and representative gnathostomes, as proposed by Onimaru et al.. Purple, orange and light blue bars represent the pharyngeal mesoderm (ph), the anterior LPM (ALPM) and the posterior LPM
(PLPM), respectively. Double‐headed arrows indicate the somatic mesodermal layer. Distribution of Tbx4/5 (purple) in amphioxus and lampreys, Tbx5 (blue) in gnathostomes, and collinear Hox genes (green bars) in lampreys and gnathostomes are indicated in each embryo. vmp, ventral mesoderm posterior to the pharynx.
Schematic modified from Onimaru et al. and Tanaka . Models in panels A–F were proposed by Jarvik ; Tabin and Laufer ; Thorogood and Ferretti ; Ruvinsky and Gibson‐Brown , and Onimaru et al., respectively.

Figure 2. Expression and regulation of 5'Hox genes. (a) Schematic illustration of the early (top) and late (bottom) phases of 5'Hoxa (magenta) and 5'Hoxd (blue) genes in mouse forelimb buds. Expression patterns of 5'Hox genes were re‐drawn and modified after Dolle et al.. (b) Model for the regulatory evolution of HoxA and HoxD genes during the fin‐to‐limb transition proposed by Woltering et al.. Coloured shapes located in the 5′ and 3′ regions of each Hox gene cluster (black rectangle) indicate enhancers, and arrows
indicate the interactions between these enhancers and the Hox cluster. In fish fins, this interaction may pattern the proximal (red) and distal (orange) skeletal elements. In tetrapod
limbs, new enhancers have been acquired or existing ones were modified, and thereby, a novel and more distal autopodial identity
may have evolved. Redrawn and modified after Woltering et al. 2014 published by PLOS licensed in accordance with the Creative Commons Attribution (CC BY) license.

Adachi N, Robinson M, Goolsbee A and Shubin NH (2016) Regulatory evolution of Tbx5 and the origin of paired appendages. Proceedings of the National Academy of Sciences of the United States of America 113 (36): 10115–10120.

Balfour FM (1881) On the development of the skeleton of the paired fins of Elasmobranchii, considered in relation to its bearings on the nature of the limbs of the vertebrata. Proceedings of the Zoological Society of London 1881: 656–671.

Gehrke AR, Schneider I, de la Calle‐Mustienes E, et al. (2015) Deep conservation of wrist and digit enhancers in fish. Proceedings of the National Academy of Sciences of the United States of America 112 (3): 803–808.

Gibson‐Brown JJ, Agulnik SI, Chapman DL, et al. (1996) Evidence of a role for T‐box genes in the evolution of limb morphogenesis and the specification of forelimb/hindlimb identity. Mechanisms of Development 56 (1–2): 93–101.

Lettice LA, Horikoshi T, Heaney SJ, et al. (2002) Disruption of a long‐range cis‐acting regulator for Shh causes preaxial polydactyly. Proceedings of the National Academy of Sciences of the United States of America 99 (11): 7548–7553.

Minguillon C, Gibson‐Brown JJ and Logan MP (2009) Tbx4/5 gene duplication and the origin of vertebrate paired appendages. Proceedings of the National Academy of Sciences of the United States of America 106 (51): 21726–21730.

Mivart SG (1879) On the fins of elasmobranchii. Transactions of the Zoological Society of London 10: 439–484.

Schneider I, Aneas I, Gehrke AR, et al. (2011) Appendage expression driven by the Hoxd Global Control Region is an ancient gnathostome feature. Proceedings of the National Academy of Sciences of the United States of America 108 (31): 12782–12786.

Shapiro MD, Bell MA and Kingsley DM (2006) Parallel genetic origins of pelvic reduction in vertebrates. Proceedings of the National Academy of Sciences of the United States of America 103 (37): 13753–13758.

Thacher JK (1877) Median and paired fins, a contribution to the history of vertebrate limbs. Transactions of the Connecticut Academy of Arts and Sciences 3: 281–310.

Thewissen JG, Cohn MJ, Stevens LS, et al. (2006) Developmental basis for hind‐limb loss in dolphins and origin of the cetacean bodyplan. Proceedings of the National Academy of Sciences of the United States of America 103 (22): 8414–8418.

Tulenko FJ, McCauley DW, Mackenzie EL, et al. (2013) Body wall development in lamprey and a new perspective on the origin of vertebrate paired fins. Proceedings of the National Academy of Sciences of the United States of America 110 (29): 11899–11904.

Zakany J, Fromental‐Ramain C, Warot X and Duboule D (1997) Regulation of number and size of digits by posterior Hox genes: a dose‐dependent mechanism with potential evolutionary implications. Proceedings of the National Academy of Sciences of the United States of America 94 (25): 13695–13700.